Transmission line transducer



. J. EVANS TRANSMISSION LINE TRANSDUCER Filed Nov. 8, 1944 sept; 3o, 1947.

llllllllllF/ sept. 3o, 1947. J EVANS 2,428,272

TRANSMISSION LINE TRANSDUCER Filed Nov. s, y1944 s sheets-sheet 2 l JZ 5E IE INVENTOR ATTORN EY sept. 3o, i1947.

J. EVANS TRANSMISSION LINE TRANSIVJUGER Filed Nov. 8, 1944 3 Sheets-Sheet 5 INVENToR c/oA//v fc4/v5.

ATTORNEY UNITED STATESl ATENT OFFICE TRANSMISSION LINE TRANSDUCER `lohn Evans, Kingston, N. J., assigner to Radio l Corporation of America, a corporation of Delaware Application November 8, 1944, Serial No. 562,551

18 Claims. (Cl. 179100.4)

My present invention relates generally to reference to the following description, taken in transducers, and more particularly to improved connection with the drawing, in which I have phonograph pickup units and microphone deindicated diagrammatically several circuit orvices. ganizations whereby my invention may be carried One of the main objects of my present inveninto effect. tion is to provide a phonograph pickup unit In the drawings: which possesses a relatively low inertia, and Fig. 1 iS a circuit diagram of a phonograph which essentially comprises transmission line pickup system constructed in accordance with resonant circuits. my invention;

Another importent object of my invention is to 1o Fig. 2 shows the frequency response curve 0f provide a frequency modulation (FM) microthe discriminator circuit of the system of Fig, l; phone system utilizing transmission line circuits Fig. 3 is a plan view, partly in section, of a tone thereby permitting the conversion of sound enarm constructed in accordance with my present ergy to electrical energy over a wider frequency invention; range than is possible with magnetic or pure l5 Fig. 4 is a sectional view of the tone arm taken capacitive devices. along line 4 4 of Fig. 3 looking in the direction Another important object of my invention is Of the arrOWS;v to provide a mechanical support, such as the Fig. 5 is a sectional view of the tone arm taken tone arm of a pickup unit or the vertical support along the line 5 5 of Fig. 4 looking in the ditube of a microphone, which has a double func- 2o rection of the arrows;

tion in that it is constructed to act as a trans- Fig- 6 iS a partially Schematic View irl eleVad mission line type of frequency discriminator. tion of a microphone constructed in accordance Still another object of my invention is to prowith my invention; and vide a pair of concentric line resonators tuned Fig. 7 iS a pian View 0f the micrODhone systo diiferent, but closely adjacent, frequencies; 2o tem shown in Fig. 6. one of the resonators functioning as the tank Referring 110W t0 the accompanying drawings, circuit of an ultra-high frequency oscillator, the wherein like reference numerals in the various other resonator acting as a discriminator for frelgureS indicate Similar circuit elements, I have quency modulated oscillations created by virtue SllOWrl irl Fig. 1 the fundamental circuit 0f my of frequency change of the tank resonator in improved System for translating reactance response to variations of the reactive element changes into electrical currents representative of of a transducer, such changes. While the specific reactive de- Another object of my invention is to provide a Vice SllOWrl irl Fig. 1 is a condenser including a pair of concentric line resonators each function- Stylus-actuated electrode, it iS t0 be clearly urling as a, tuned circuit of a transducer system, the derstood that myL invention is not restricted to resonators additionally acting as the mechanical a stylus for actuation of the adjustable electrode. support of a variable e1ement of the transducer. nor to a condensve reactance. Generally speak- Yet another object of my present invention is ing. my system may be employed wherever a to provide an FM phonograph pickup unit which physical force is to be indicated or represented, is self-contained but minus the iilament and 4o and where the force may be emplOyed t0 displace anode current supplies, permitting its use with the mobile element of the reactive device therepresent record reproducers with no major by to Vary the frequency 0f the aSSOCiatecl 0S- changes in the existing amplier circuitscillator. Such frequency variations may then be Still other objects of my present invention are detected, and the detected currents indicated in to improve generally the simplicity and efficiency any desired manner.

of phonograph pickup units and microphones, In the circuit of Fig. 1 the numeral I denotes andfmore especially to provide frequency moduthe Stylus of a phOIlOgraph pickup unit. The lation transducer systems which are not only restylus or needle I may be mechanically coupled liable in operation but are economically manuin any suitable manner to the mobile electrode factured and assembled. 2 of a Condenser C1. The dotted line 3 is to be The novel features which I believe to be charunderstood as representative of any suitable couacteristic of my invention are set forth with parpling which will cause displacements of electicularity in the appended claims; the invention trode 2 in response to motion of the stylus i. itself, however, as to both its organization and For example, and merely to illustrate my system,

method of operation will best be understood by the stylus point could be riding through the 3 grooves of a phonograph record mounted on a suitable turntable (not shown to preserve simplicity of disclosure).

The condenser C1 is electrically associated with an inductance L1 of the general form of a concentric line resonator. Those skilled in the art of ultra-high frequency communication are fully aware of the construction and design of such a line. Briefly, the line consists of a metal tube, shell or cylinder 4 which is at ground potential. The shell is provided with a concentric, axiallylocated metal rod 5 which is also at ground potential. The condenser C1 isA connected between the upper ends of shell 4 and vrod 5.V Hence, the variable condenser provides a means for adjusting the frequency of the transmission line resonant circuit L1C1. Obviously, displacements of stylus I Will result in corresponding variations of the capacitance of condenser C1. The frequency of concentric line resonator L1C1 Will be varied in accordance with the capacitance variations of C1.

Preferably the effective electrical length of resonator L1 is chosen to be one quarter of the normal operating wavelength. Such an electrical length provides an inherently stable circuit. Further, the impedance along the concentric line resonator L1 is zero at the groundend, and increases to a maximum to the point Where the stylus l is ailixed. This is a practical advantage, since small stylus displacements will effect large impedance changes. Hence, the stylus and associated condenser may be constructed of very light material, and in effect provide a low inertia pickup unit. It is assumed, merely by way yof specific explanation and illustration, that the normal or operating frequency of resonator L1C1l is 400 megacycles (mc). It is to be clearly understood that the normal frequency of LiCi can be chosen from a wide range of ultra-high frequencies, e. g., 20E) to 500 rnc. The choice of operating frequency will depend upon the j physical length of shell 4 desired. At 300 me., for example, the electrical length of the inner conductor or rod 5 would be one fourth of a meter, or 9.9 inches. The physical length of the shell 4' could be made longer than the latter length without affecting the electrical length by having the end of shell 4 project a predetermined distance beyond the end of conductor 5. The concentric line resonator functions as the tank circuitof an oscillator.

The oscillator tube 6 is preferably one of the miniature type; i. e., the so-called acorn type. It is desirable to use a tube of relatively high amplification factor. The indirectly-heated cathode T, which mayr be of the' filament type if desired, is connected to ground through coil 8 and grid bias resistor 9 arranged in series. The condenser lil bypasses resistor 9 to ground, and establishes the lower end of cathode coil 8 at ground potential for radio frequency currents. The control grid H is connected by lead l2 to a suitable point on inner conductor rod 5 such that the input impedance of the tube matches the impedance of the line resonator between the tapping point and ground. The plate I3 is connected to a direct current potential point +B of suitable positive value; plate i3 is connected to ground by condenser I4` for radio frequency currents. Y

The oscillations are produced at the normal frequency of the concentric line tank circuit L1C1. It will be noted that the plate is at ground potential as respects radio frequency potentials, and that radio frequency voltage developed across cathode coil 8 is fed back to the tank circuit of tube 6. The oscillations thus produced are of relatively high amplitude, and are stable as to frequency by virtue of the degenerative effect on the cathode coil 8 of the grounded plate. The ultra-high frequency oscillations may be produced in any other way; I desire to make it clear that there may be used any other specific method of producing stabilized oscillations. The outer shell 4 being grounded inherently functions as a shield. It is desirable to use for the material thereof a metal of high electrical conductivity and low loss. For example, a fibrous shell could be employed with metal coatings provided over the inner and outer surfaces thereof. Such a shell would have a low inertia in addition to the above characteristics. The inner conductor 5 could be similarly constructed.

The oscillatory output of tank circuit L1C1 is frequency modulated (FM), since the variations in capacitance of condenser C1 cause variations in the frequency of the tank circuit. The extent of frequency variation or deviation of F1, the stable normal oscillator frequency, depends on the amplitude of the stylus displacement. Since the latter is responsive to the grooves of a sound record, it follows that the audio frequency amplitude determines the extent of frequency swings of the circuit L1C1. The rate of frequency deviation is a function of the audio frequencies per se. I use the generic term angle modulated to denote the fact that the modulation of the oscillator output may be either in frequency or phase. The overall maximum frequency swing of the FM oscillations is dependent on the L/C ratio of the tank circuit, and can be as wide as 50 kilocycles (kc).

The FME oscillations are detected by a discriminator-rectier network comprising a second concentric line resonator which is` coupled to the tank circuit L1C1. The latter feeds amplitude modulated oscillations to a detector tube. The second resonator consists of an outer metal shell l5 whose lower end isgrounded. The second concentric resonator has an electrical length such that inductance L2 of the resonator tunes with condenser C2 to ak frequency higher or lower than the normal oscillating frequency. The discrimnator circuit LzCz, in the form of the concentric line resonator, may be constructed in the same manner as resonator L1C1, except that the condenser C2. would be adjusted so as to have the frequency of LzCz suitably different from that of L1C1. The line Il couples suitable points on the inner conductors 5 and I6 so as to match the impedances of the tuned circuits. Once chosen, the magnitude of Cz is not changed in the particular unit, except for-servicing of the latter.

The detector tube I8 may be ay miniature type triode of the same type as tube 6. Indirectlyheated cathode I9 is connected to ground by grid bias resistor 2-3, bypassed to ground for radio frequencies by condenser 24. Grid 2| is connected by coupling lead 22 to a suitable point on conductor I6 such that the input impedance of tube i8 matches the tapped section of the input circuit. The plate 20 is connected to the +B voltage point of a suitable direct current source through load resistor 2'6 whose magnitude may be about 50,000 ohms, the lower end of the load being bypassed to ground by condenser 25. The modulation frequency, audio in the case of records, voltage is transmitted through coupling condenser 21 to any suitable audio frequency amplifier circuit.

The curve in Fig. 2 shows the form of response curve desired for discriminator circuitLzCz. The curve was secured by plotting frequency in megacycles as abscissae against A. F. voltage produced across resistor 26 as ordinates. The peak frequency of the curve in Fig. 2 is the normal resonant frequency of the circuit LeC'z. The physical lengths of outer shells 4 and I5 will be the same, but the electrical lengths of the two resonators will be suiiiciently different to provide the frequency displacement specified. The normal oscillator frequency F1 will be selected so as to fall at the midpoint of a linear portion of one of the flanks of the response curve. For example, the peak frequency of circuit L2C2 may be 400.08 mc., and the normal oscillator frequency F1 may be located at 400 mc. The extreme ends of the frequency swing F2 and Fs would then be 400.025 mc. and 399.075 mc. respectively, assuming an overall frequency of 50 kc.

The FM oscillations applied to circuit LzCz will develop corresponding amplitude-variable energy by virtue of the sloping flank of the response curve. The variable amplitude energy will be detected by detector tube IB, since the resistor 23 provides sufficient grid bias to permit the tube to act as a detector of the plate rectification type. The output voltage across resistor 26 will vary between extreme values of E+ and E- corresponding to frequency extremes F2 and F3 respectively. Of course, the invention is not restricted to the specific frequencies mentioned. Furthermore, the oscillator frequency F1 could be located at a similar midpoint on the opposite flank of the response curve in Fig. 2. In general, if one of the two circuits L1C1 or LzCz has an electrical length of 1A of a wavelength, the other circuit would have an electrical wavelength a suitable amount plus or minus different therefrom.

In Figs. 3, 4 and 5 I have shown an embodiment of a tone arm and pickup unit constructed in accordance with my invention. The outer metal shells 4 and I5 of the oscillator tank circuit and discriminator circuit respectively are each in the form of horizontal cylinders. The left ends of the shells 4 and I5 are provided with dielectric closures 30 and 3| respectively. The right ends of the shells 4 and I5 are provided with metallic closures 32 and 33. As shown in Figs. 3 and 5 the shells 4 and I5 are in direct contact, a common opening 34 being provided in the shell walls at a predetermined point along the line of contact of the shells. The coupling conductor I1 f passes through opening 34 so as electrically to connect the inner conductors 5 and I6.

The inner conductors 5 and I6 are in conductive contact with closures 32 and 33 respectively at the centers thereof. They may be secured thereto by soldering, or by any suitable means for providing a secure and rigid joint. The opposite ends of conductors 5 and I6 pass through the centers of the respective insulation closures 30 and 3l into a head, or forward casing, housing the electrodes of condensers C1 and C2. If desired, the coupling between the two resonators may be field coupling. By removing conductor I1 and replacing opening 34 by an extensive slot common to both shells, such field coupling is readily provided.

Condenser Ci consists of the mobile electrode, or plate, 2 and stator or fixed electrode 2. The stator electrode 2 is secured tothe left end of conductor 5 extending into the interior of casing 35. The latter has a generally rectangular crosssection, and is preferably composed of the same metallic material as shells 4 and I5. The electrode 2' is preferably provided with a flange 36 so as to permit ready affxing to the end of conductor 5. The flange 36 may be soldered or bolted to the conductor 5. The mobile electrode 2 is secured to the upper end of a pivoted vertical rod 31 whose lower end is provided with stylus I. The casing 35 is provided with a slot 38 through which the lower end of rod 31passes.

The slot 38 permits motion of the rod 31, in response to displacement of stylus I due to scanning the record grooves, relative to pivot point 39.- The latter may be provided by a support arm 40 whose free end acts as a bearing for the rod 31. The displacements of stylus I are translated into motion of mobile electrode 2 with respect to stator 2. It is to be clearly understood that this construction of the variable condenser C1 and its mechanical coupling to stylus I is purely illustrative. Any other suitable construction may be employed to translate motion of the stylus into corresponding variable capacitance of a condenser.

The fixed condenser C2 consists of a pair of electrodes or metal discs 40 and 4I. The electrode 40 is rigidly secured to the end of conductor I 6. The electrode 4I is normally spaced from the electrode 40 by a predetermined distance. To permit factory adjustment, or service adjustment, the disc 4I may be affixed to the inner end Y of a threaded stem 42 whose exposed head is provided with an adjustment slot. In this way the capacitance of Cz may be adjusted easily and simply in order to secure a desired and suitable peak frequency for discriminator circuit LzCz.A

The condensers C1 and C2 are completely housed within the shielding casing 35. The latter is in conductive contact with shells 4 and I5 whereby the inductive and capacitative elements of both tank and discriminator circuits are shielded from external interference. lThe pickup unit, the tank and discriminator circuits are, moreover, a common mechanical system wherein the shells of the concentric resonators function as mechanical supports rfor the pickup unit. The tubes 6 and I8 are housed within a metallic casing 5I] of circular cross-section. The casing 50 is snugly fitted over the right ends of shells 4 and I5. The opening of casing 50 should conform to the circular configuration of shells 4 and I5 as closely as possible.

In Fig. 3 these two tubes are schematically represented, the grid leads I2 and 22 from respective tubes 6 and I8 to respective conductors 5 and I6 being shown. The grid leads I2 and 22 are passed through suitable bores in the closure discs 32 and 33 respectively. The current leads to the tubes are not shown in Fig. 3. However, Fig. 4 shows the manner of mounting the physical tubes (tube 6 specifically) on the base of casing 50. The energizing current leads 5I are shown in Figs. 4 and 5. It will be seen that the power leads to the filament and plate are run up through the interior of metallic support post 52 which is secured to any suitable chassis 53;

The metallic support post 52 is hollow, and is grounded by virtue of its rigid coupling to metallic chassis 53. The horizontal shells 4 and I5 may be secured to the upper end of post 52 in any desired fashion as by soldering, welding and the like. From Fig. 5 it is seen that the upper end of support post 52 provides a fulcrum point forthe tone arm.v Briefly, it is desired to couple the tone arm to the support post in such a manner as to permit suitable motion of the tone arm during record reproduction. Those skilled in the art of constructing phonograph tone arms will readily be aware of any suitable means for providing a universal coupling joint between the tone arm unit and the support post 52. For this reason the mechanical details of the tone arm support post are not shown.

The energizing current leads are passed through the top opening of post 52, and then along the space underneath the line of contact between shells 5 and l5. It will be understood that the lower lip of the forward opening of casing 50 will be constructed so as to `permit the leads 5| to be run into the interior of casing 5D, but along the space adjoining the line of contact of the rear ends of shells 50` and l5. :It is not believed necessary to show the detailedconstruction of the bases for receiving the prongs of tubes 6 and I8. It will be realized that the outer shells 4 and I5 of the concentric resonators are at ground potential by virtue of the metallic connection between the vertical support post 52 and the pair of shells. Furthermore, the casings 35 and 50, being in metallic contact with the grounded shells, are also at ground potential. In this way the entire unit functions as a shield for the circuit elements located within the interior of the front and rear casings and the shells. It will, therefore, be seen that I have provided a transmission line phonograph pickup unit which is of low inertia. Further, there is provided a self-contained unit functioning simultaneously as a mechanical tone arm and a 4pair of electrical circuits, while additionally including a section adapted to carry or support the oscillator and detector tubes. In Figs. 6 and 7 I have shown an embodiment of the invention which is essentially a transmission line microphone. In this case certain of the elements in Fig. 6 are schematically represented, while other of the elements are shown in constructional form. Still other elements are shown in section in order clearly to reveal the mechanical structure of the device. In this modification of the invention the electrical circuits involved are substantially the same as in Fig. 1, except for the fact that the condenser C1 is a voice-actuated condenser microphone.

The oscillator circuit in the case of Fig. 6 comprises the vertical concentric line resonator consisting of the outer shell BB `and the inner conductor Bl. The upper end of the inner conductor 6I terminates in the form of thefixed plate 62 of the condenser microphone. The latter is represented by the symbol C1, because it is analogous in function to the stylus-actuated condenser C1 in Fig. 1. The electrode 62 is maintained in vertical fixed position by virtue of the offset coupling 63 to theupper end of inner conductor El. As a matter of actual mechanical construction the inner conductor 6l may have the sections 63 and 62 integral therewith, but it is to be understood that the top end of shell 50 is closed by the closure disc 64. The inner conductor passes through the center thereof to provide the offset section 63.

. The second concentric line resonator comprises the vertical outer shell 65 whose physical length is the same as the physical length of shell B, and whose upper end is similarly closed by metallic disc 66. The shell 65 is provided with the axial inner conductor 61. The shells 65 and 60 are in metallic contact along the entire lengths thereof.

The inner conductors 67 and 6I are electrically coupled by the direct connection 68 therebetween. It will be understood that the electrical connection E8 is provided through common openings in the contacting surfaces of the shells, as shown in Fig. 3 for the conductor l1.

I'he condenser Cz, which is analogous in function to the condenser Cz in Figs. l and 3, is provided in this modification by utilizing the upper end of inner conductor Si as one electrode of the condenser. The adjustable second electrode of condenser C2 is provided by means of a metallic disc located within the upper end of shell 65'in spaced relation from the end of conductor 61. The numeral 59 indicates the adjustable electrode, and it will be observed that electrode 89 is provided with a threaded stem lll having a slotted head so as to permit ready adjustment of the space between electrode t9 and the electrode which is provided by the adjacent upper end of conductor Gl.

It will be noted that the upper end of conductor 6l terminates in spaced relation from the closure 65, and that the threaded stern i5 protrudes from the shell S5. The mobile or displaceable electrode of .condenser Ci is provided by a ribbon 'Il (schematically portrayed) of suitable metallic material. For example, such material could be duraluminum, although the invention is not limited in any way to such material. Those skilled in the art of constructing microphones are fully aware of the manner of constructing a condenser microphone oi the type generally represented herein by Ci of Fig. 6. The normal spacing between electrodes 'll and 52 is chosen so as to provide a normal oscillator frequency which is located on a linear portion of one of the anks of the resonance curve of the discriminator circuit. This is indicated in Fig. 2, and it is to be understood that the explanation given in connection with Fig. 2 applies equally well to the microphone circuits of Fig. 6.

The ribbon electrode Il has its opposite ends securely clamped between the clamps indicated generally Vby numerals l2 and 'i3 respectively. Clamp 'l2 is shown consisting of an arm 'Hi whose lower end is rigidly secured in any suitable manner to the closure disc Si. of shell 65. The upper end of arm 'M is located above the upper end of electrode 62. The upper end of ribbon electrode 1I is clamped securely in place between the clamping faces at the upper end of arm l5. The lower end of ribbon electrode Il is securely clamped between the clamping faces of clamp 'I3 which is supported on the closure disc 6d. It willtherefore, be seen that electrode 'il is in conductive connection with the grounded shell 65, and that electrode 62, spaced from electrode ll, is also in conductive connection with the grounded shell Gil. Obviously when sound waves impinge on the electrode il it will have a sufliciently low inertia so as readily to move with respect to electrode 62 thereby changing the frequency of the resonant tank circuit of the oscillator. The resonant frequency of the discrirninator circuit does not change, because thc capacity of condenser C2 once predetermined and set is not changed during operation.

In Fig. 6 I have shown the protective grille ,80 surrounding the entire microphone unit and the upper portions of the concentric line .resonators. rIhe protective grille Sil is partly broken away in Fig. 6. It will be understood that the grille 8i) is of suitable construction customarily employed in velocity microphones, and it is not of Fig. 1.

lcordance with the voice waves.

believed necessary to show the grille 80 in any further mechanical detail. In Fig. '7 the grille 8U has been entirely cut through so as to show the details of the concentric line resonators. In Fig. '7 the numeral 99 indicates the base or supporting casing which houses the pair of oscillator and detector tubes and the high frequency connections thereto.

The base casing or pot 9) is composed of a metallic material, and its upper surface will be provided with an opening through which shells 65 and 6U pass. The opening 9| will, of course, be shaped so as to conform to the configurations of both shells. Preferably the bottom closure discs 92 and S3 of the respective shells are in direct contact with the bottom of pot 9B, it being desirable to solder or otherwise xedly secure the shells to the bottom section.

Tubes 6 and I8 are the respective oscillator and detector tube, exactly as in the embodiment of Fig. 1. The tubes are schematically represented in Fig. 6; the connections thereto are also 'schematically shown. However, it will be understood that the connections from tubes 6 and I8 to the inner conductors 6I and 61 are the same as shown in Fig. 1, and-the various circuit components associated with tubes Il and I8 are the same as in Fig. 1. Hence, the connections and circuit components in Fig. 6 are not described in detail, but are generally related by corresponding reference numerals to Fig. 1.

The grids II and 2| are connected by respective connections I 2 and 22 to the respective inner conductors 6I and 6I. The connections I2 and 22 pass through respective openings provided in the sections of shells 60 and 65 located within the pot 9U. The connection points of leads I2, 22 and 68 to the inner conductors 6I and 6l will depend on the circuit impedances to be matched. The output load resistor 261s in the plate circuit of tube I8, and it will be understood that pot 90, as well as shells 65, 60 and protective grille 80, will be at ground potential. The oscillator tube 6 includes coil 8 in its cathode circuit in series with grid bias resistor 9 to ground, condenser I bypassing resistor 9 to ground. The resistor 90 in the plate circuit of tube 6 Vis a hum filter. The energizing connections to the tube elements of the oscillator and detector tubes are indicated. y

The microphone system of Fig. 6 operates very much the same as the record reproducer system The concentric line resonator 6D, 6I is tuned by suitable spacing of electrodes 'Il and 62 (condenser C1) to the normal oscillator frequency F1. Reference isv again made to the resonance curve of Fig. 2 to illustrate that the oscillator frequency F1 of Fig. 6 will be set to fall at the middle of the linear section of a flank of the discriminator circuit resonance curve. The concentric line resonator 65, 61 is tuned by proper adjustment of electrode 69 to the desired peak frequency of the resonance curve. The frequency values suggested in Fig. 2 may be employed.

Variation of the spacing between electrodes 'Il and 62 of C1 will cause similar variations in the frequency of the oscillator tank circuit. Voice Waves will vary the capacitance of C1 in accordance with themodulation of the waves, and the oscillator frequency will vary in ac- The frequency variations between limits of F2 and F3, as explained in connection with Fig. 2, will result in corresponding variable amplitude high frequency l0: waves being applied to detector grid 2|. The detected audio Voltage across resistor 26 may then be suitably amplified and reproduced. It will be observed that the outer metallic surfaces of the microphone system of Fig. 6 are at ground potential. Hence, they function as a suitable shielding means. Again, the oscillator tank and discriminator circuits function as the mechanical support means for the condenser microphone elements.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent; to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention.

What I claim is:

1. Ina transducer system, a pair of electrically coupled concentric line resonatorsnormally tuned to adjacent frequencies, means directly coupling the inner conductors of said resonators,`one of said resonators including a condenser having ay mobile electrode subject to physical displacements to be indicated, an oscillator tube including said one resonator as the tankcircuit thereof, and a detector tube including the second resonator as the input circuit thereof.

2. In combination, a pair of coupled concentric line resonators tuned to different frequencies located on the resonance curve of one of the resonators, said resonators being in contact along the lengths thereof, an adjustable reactance device electrically connected with one resonator at one end thereof to vary the frequency thereof at an audio rate, and detector means coupled to the second resonator for indicating said frequency Variations.

V3. In a phonograph system, a tone arm consisting solely of a pair of coupled concentric line resonators tuned to respective frequencies separated by a predetermined frequency value, a variablecondenser pickup electrically connected to one resonator, one of said resonators having an electrical length which is substantially a quarter wavelength long at the operating frequency, lan oscillator tube including one of said resonators as the tank circuit thereof, and a detector tube including the second resonator as the input circuit thereof.

4. A microphone system comprising a pair of coupled concentric line resonators in contact'.

along the lengths thereof, said resonators being tuned to closely adjacent different frequencies, one resonator having an electrical length which is a quarter Wavelength long at the operating frequency, a condenser including a mobile electrode mounted at the top end of one resonator, and respective oscillator and detector tubes for said two resonators, and a shielding casing for said tubes at the lower ends of said resonators.

5. In a record reproducing system, ran ultrahigh frequency oscillator including a concentric line tank circuit, means including a stylus-responsive element for varying the frequency of the tank circuit, a detector having a concentric line resonator input circuit, a lead directly coupling the inner conductors of the two resonators, and means tuning said input circuitA to a frequency differing from the normal frequency of the tank circuit by an 'amount such as to permit the input circuit to act as a frequency discriminator.

6. In combination, a high frequency oscillator system including a concentric line resonator as a tank circuit, a condenser for tuning said tank circuit to a predetermined normal high frequency, said condenser having a mobile electrode, a detector including a second concentric line resonator acting as an input circuit, means directly coupling the inner conductors of said resonators, and means for tuning said second resonator to a predetermined frequency such that said normal oscillator frequency falls at an intermediate point of the flank of the resonance curve of said detector input circuit.

7. In combination, a high frequency oscillator system including a concentric line resonator as a tank circuit, a condenser for tuning said tank circuit to a predetermined normal high frequency, said condenser having a mobile electrode, a detector including a second concentric line resonator acting as an input circuit, means directly coupling said resonators, means for tuning said secondV resonator to a predetermined frequency such that said normal oscillator frequency falls at an intermediate point of the flank of the resonance curve of said detector input circuit, said resonators each comprising a metallic shell, said shells being in direct contact, and each shell having a respective axially-located inner conductor rod, said direct coupling means being connected between suitable points of said inner rods.

8. In combination, a high frequency oscillator system including a concentric line resonator as a tank circuit, a condenser for tuning said tank circuit to a desired high frequency, said condenser having a mobile electrode, a detector including a second concentric line resonator acting as an input circuit, means electrically coupling said resonatorsmeans for tuning said second resonator to frequency such that said normal oscillator frequency falls at an intermediate point of the flank of the resonance curve of said detector input circuit, one resonator having an electrical length of one quarter wavelength at the operating frequency thereof, and said oscillator resonator including a metal shell whereby it functions as a mechanical support for said condenser.

9. In a record reproducer system, a pair of concentric line resonators, each resonator comprising an outer metal shell and an axially-located inner conductor rod, respective tuning condensers connected between the inner rod and outer shell of each resonator, said condensers tuning the resonators to adjacent high frequencies, one resonator being electrically a quarter wavelength long, one of said condensers including a stylus-actuated electrode, and means directly connecting said inner rods.

10. A phonograph tone arm comprising a pair of concentric line resonators of substantially equal physical length and in contact along the entire lengths thereof, the effective electrical length of one resonator being a quarter wavelength at the operating frequency, and the electrical length of the second resonator being such that the resonant frequency thereof is different from that of the one resonator, and a variable reactance pickup electrically connected to one resonator.

11. In a microphone system, a pair of electrically coupled concentric line resonators tuned to closely adjacent frequencies, a short lead directly coupling the inner conductors of said resonators, one of said resonators including a reactance having a mobile electrode subject to physical displacements to be indicated, an oscil` lator including said one resonator as the tank circuit thereof, and a detector including the second resonator as the input circuit thereof.

12. In combination, in a record reproducer, a pair of coupled concentric line resonators tuned to different frequencies located on the resonance curve of one of the resonators, an adjustable condenser device electrically connected with one resonator to vary the frequency thereof, said resonators being in direct contact and concurrently acting -as the tone arm, and detector means coupled to the second resonator for deriving audio signals from said frequency variations.

13. In a record reproducer system, a tone arm consisting of a pair of rigid elongated concentric line resonators tuned to respective frequencies separated by a predetermined frequency value and in direct contact, said resonators each having an effective electrical length which is substantially a quarter wavelength, and a reactance pickup device coupled to one resonator.

14. In a record reproducing system, a high frequency oscillator including a concentric line tank circuit, means including a stylus-responsive element for varying the frequency of the tank circuit to produce frequency modulated oscilla-l tions, a, detector having a concentric line resonator input circuit coupled to said oscillator, a short lead directly coupling the inner con-4 ductors of said concentric line resonators, and means tuning said input circuit to a frequency differing from the normal frequency of the tank circuit by an amount such as to permit the input circuit to act as a frequency discriminator.

15. In combination, a high frequency oscillator system including a concentric line resonator as a tank circuit, a condenser for tuning said tank circuit to a desired frequency, said con-v denser having -an adjustable electrode, a detector including a second concentric line resonator acting'as an input circuit, means coupling the inner conductors of said resonators, and means for tuning said second resonator to a frequency such that said oscillator frequency falls at an intermediate point of a linear section of the flank of the resonance curve of said detector cuit.

16. In a microphone system, a pair of electrically coupled rigid and elongated transmission line resonators tuned to closely adjacent frequencies and in direct contact along the lengthsV thereof, one of said resonators including Aa, reactance having a mobile ribbon electrode, an oscillator tube including said one resonator as the tank circuit thereof, and a detector tube including the second resonator as the frequency discriminator input circuit thereof.

17. In combination, a pair of directly coupled transmission line resonant circuits tuned to dif,- ferent frequencies located on the resonance curve of one of the resonant circuits, an adjust- -able reactance device electrically connected with one resonant circuit to vary the frequency thereof, an oscillator including said one resonant circuit as the tank circuit thereof, and means coupled to the second resonant circuit for indicating said frequency variations. Y

18. In a transducer system, a support arm consisting solely of a pair of rigid elongated transmission line circuits tuned to respective frequencies separated by a predetermined small frequency value, said transmission lines-being in direct contact, one of said circuits having an ellective electrical length which is substantially input cir- 2,428,272 13 Y 14 a quarter wavelength at the operating frequency, UNITED STATES PATENTS and a variable reactance pickup connected to one of the elongated transmission line circuits. Number Name Date JOHN EVANS. 2,361,634 Koch oct. 31, 1944 2,277,638 George Mar. 24, 1942 REFERENCES CITED 2,303,338 Pray Dec. l, 1942 The following references are of record in the le of this patent: 

